The present invention relates to an abnormality detection circuit for a motor in which causes of abnormality of the motor can be precisely specified.
Although, in many cases, the motor is used for an electronic device or a control device, the motor can not be rotated and operated normally when the abnormality occurs in a driving circuit of the motor. For example, when the abnormality occurs in the driving circuit of a copying sheet conveyance motor in an electrophotographic copier (hereinafter, which will be called a copier), the copying sheet can not be conveyed.
Conventionally, whether the motor is rotated or not, can be easily detected from an output of an encoder provided on a rotational shaft of the motor. However, as a result, even when it is detected that the motor is not rotated, the cause of abnormality can not be detected, and therefore, a display showing an error is simply shown.
In the case of the copier, users require a serviceman to repair the copier when an error massage is displayed. However, the serviceman can not specify only from the message which portion of the copier is defective, and accordingly, he should carry various parts for replacement such as a power circuit of a motor, a printed circuit board for driving, or a motor. Usually, a method is used in which a part, which is likely defective, is successively replaced with a new one in the manner that, at first, the power circuit of the motor is inspected, and next, the printed circuit board for driving is replaced with new one.
However, a large part of the repairing operation is conducted depending on experience and intuition of the serviceman, and in the case of a serviceman, whose experience is small, the repairing operation can not be smoothly conducted, and repair parts losses are large.
In view of the foregoing, the first object of the present invention is to specify precisely the cause of abnormality when the abnormality occurs in a portion including the motor, and to cope with the abnormality quickly and accurately.
Further, the second object of the present invention is to judge whether the cause of abnormality is included in the motor itself, or if a motor drive element is defective, when the motor is not rotated, and to cope with the abnormality quickly and accurately.
Further, the present invention relates to an abnormality detection device of rotation speed of a motor which is used for an electrophotographic copier or the like.
There are many electronic devices and industrial devices for which motors are used. Various methods for detecting the rotational speed of the motor are widely known, and a method in which a rotary encoder is used is among them. The method is a method in which a disc, which is called a rotary encoder, having a large number of slits is provided on the rotational shaft of the motor; passage of light through the slits of the disc, which is rotated, is optically detected and converted into electric pulses; and the rotational speed of the motor is detected from the number of pulses per a unit period of time.
Conventionally, in order to detect abnormality of the rotational speed of the motor, a method is known in which a clock signal having a high frequency is generated on the one hand; and a variation of the clock signals corresponding to one pulse of a group of pulses is obtained from the rotary encoder.
In a copier, when an image formed on a photoreceptor on a rotational drum is transferred onto a copying sheet, the copying sheet fed from a cassette is stopped once and stands-by at the place in order to make a fore end of the image coincide with that of the copying sheet, and the copying sheet is fed again in timed relation with the movement of the image. In order to conduct the above described operation, a mechanism is used in which a clutch is operated once, and a feeding motor which is always rotated, is stopped once. In the above described case, in the method in which the abnormality of the rotational speed of the motor is detected by a variation of the number of clock pulses, the case where the rotational speed of the motor is decreased when the clutch is operated is judged as the abnormal speed, which is a problem.
Accordingly, in Japanese Patent Publication Open to Public Inspection No. 59323/1986, a control method for abnormal speed of a motor has been proposed in which: a driving-current of the motor is marked and whether the driving current is abnormal or not, is checked; when the driving current is abnormal and the abnormal condition is continued for a longer period of time than a predetermined period of time, or the abnormal condition occurs predetermined times, it is judged that the motor speed is abnormal; and the rotation of the motor is gradually stopped.
Due to the foregoing method, because the clutch is instantaneously operated, the decrease of the rotational speed by the clutch operation is not erroneously detected as abnormal speed. However, a counter to detect that an abnormal current is continued for a predetermined period of time, is necessary. Further, because it takes a long period of time until the current is judged to be abnormal, the correspondence to the next operation by which the motor is stopped, or the abnormality is displayed, is likely to be late.
A control method, in which a PI (proportional plus integral) action is used, is widely known as a motor speed control method. The control method using the PI action is a control method in which a speed command signal to control the rotational speed of the motor is caused to be a sum of a controlled variable (proportional term) which is proportional to a difference between a target rotational speed V.sub.S and an actual rotational speed V and a controlled variable (integral term) which is proportional to a time integral of the difference. Giving consideration to the integral term of the speed command signal used in the PI action control, it can be found that an amount of variation of the integral term is remarkably different for an instantaneous decrease of the rotation speed of the motor caused by the clutch operation when compared with the case of a speed variation caused by an abnormality of the actual speed.
Accordingly, the third object of the present invention is to detect an abnormality of the rotation speed of the motor by a simple circuit without using a counter or the like in the speed control of the motor which is operated in accordance with the PI action.